1. Field of the Invention
The present invention relates to the realization of a fast diode capable of withstanding a high reverse voltage by using a series connection of several diodes.
2. Discussion of the Related Art
Two solutions are conventionally available when a high voltage diode is desired to be made. A first solution consists of sizing the diode by manufacturing, that is, manufacturing a diode having an avalanche voltage adapted to the high reverse voltage that the diode is to withstand. A second conventional solution consists of connecting several diodes in series so that the sum of their respective avalanche voltages corresponds to the avalanche voltage desired for the high voltage diode. This second solution is often preferred when the diode switching time is desired to be enhanced, that is, when a low switching time upon the switching from the forward on-state to a reverse off-state is desired.
FIG. 1 illustrates the characteristic of the recovery time trr according to the forward voltage VF of the diode. Several characteristics a, b, c have been plotted for diodes having different avalanche voltages. For example, curve a corresponds to a diode having an avalanche voltage Vrma of approximately 1200 volts. Curve b corresponds to the characteristic of a diode having an avalanche voltage Vrmb on the order of 600 volts and curve c corresponds to the characteristic of a diode having an avalanche voltage Vrmc of approximately 300 volts.
FIG. 1 shows that the recovery time of a diode depends on its avalanche voltage and on its forward voltage drop. The smaller the forward voltage of the diode, which is advantageous since it limits the dissipated power when the diode is on, the longer the recovery time.
This drawing also shows that, when a short recovery time is desired (for example, trr0 in FIG. 1), this cannot be obtained with any diode and this requires choosing a diode having a sufficiently small avalanche voltage. If the diode has to withstand a higher reverse voltage, several diodes generally have to be connected in series to obtain the desired reverse voltage withstand by adding the avalanche voltages of these series-connected diodes. Two diodes having a same 300-volt avalanche voltage will for example be used to form a fast diode having a 600-volt avalanche voltage. Of course, the forward voltage drop of the obtained high voltage diode corresponds to the sum of the forward voltage drops of the series-connected diodes.
FIG. 2 illustrates the series connection of two diodes D1, D2 intended to behave as a high voltage diode. Conventionally, a capacitor C1, C2 is placed in parallel on each diode to form a dynamic balancing network. The function of capacitors C1 and C2, the values of which are empirically determined and are on the order of several hundred picofarads, is to minimize the influence of an imbalance between the recovered charges of the two series diodes.
The recovered charges are defined as being the time integration of the reverse current flowing through the diode at the time of its blocking (becomes non-conducting). This definition is illustrated by FIG. 3 which schematically shows the current I in a diode according to time at its blocking. It is assumed that at a time t0, the biasing across a diode is inverted by forcing the current to vary with a constant slope. Thus, current I decreases from a value IF corresponding to the direct current flowing through the diode before its blocking to a negative value Irm, before increasing to become zero. The duration (between times t1 and t2) during which the diode conducts a reverse current is called the reverse recovery time trr. This duration is necessary to the evacuation of the remaining stored charges when the direct current becomes zero (time t1). These remaining charges are called the recovered charges. The amount of recovered charges depends on several factors and, in particular, on the current decrease slope at the diode blocking. The other parameters acting upon the amount of recovered charges are, in particular, the current IF flowing through the diode before it becomes non-conducting, the operating junction temperature, and the voltage applied in reverse to the diode for its blocking.
A diode, at its switching off, can as a first approximation be modeled by two capacitors in parallel, one representing the diode junction capacitance and the other representing a capacitance linked to recovered charges Qrr.
FIG. 4 shows an electric diagram equivalent to the diagram of FIG. 2 at the end of the switching, during the blocking.
It is assumed that diodes D1 and D2 have respective recovered charges different from each other by a value .DELTA.Qrr, the recovered charge of diode D2 being greater than that of diode D1. In FIG. 4, both diodes have been modeled at the time when diode D1 has evacuated all its recovered charges and where there only remains for diode D2 to evacuate the excess of recovered charges .DELTA.Qrr that it has with respect to diode D1. Diode D1 thus is modeled by its junction capacitance Cj1 and diode D2 is modeled by its junction capacitance Cj2 in parallel with a capacitance Crr linked to the excess of recovered charges AQrr that diode D2 still has to evacuate. Indeed, the current which flows in reverse through the series-connected diodes is the same, so that the difference of recovered charges between the two diodes translates, at the end of the blocking, as the presence of a capacitor of recovered charges in parallel on the junction capacitor of a single one of the two diodes.
In the absence of capacitors C1 and C2, when diode D1 has eliminated all its recovered charges, it is in the blocked (non-conducting) state while the second diode still contains recovered charges, that is, it is reverse-conductive. All the reverse voltage is then across the first diode. Since this first diode is chosen with an avalanche voltage slightly greater than half the applied reverse voltage, it thus starts an avalanche.
Thus, in prior art, it has been considered as necessary to provide external balancing capacitors C1 and C2 to minimize the influence of capacitance Crr linked to the recovered charges in the final diode blocking phase. For this purpose, two capacitors of significant value are generally provided, to make capacitances Cj1 and Cj2+Crr negligible as compared to the capacitances of capacitors C1 and C2 (FIG. 2).
An example of application in which series diodes are often used to improve the switching time of a high voltage diode system is the field of switched-mode power supplies. The search for the shortest possible switching time trr here is due to the desire of minimizing energy losses by dissipation at the switchings.
The presence of balancing capacitors adversely affects the miniaturization and increases the cost of switched-mode power supplies and other systems in which a fast high voltage diode is desired to be made by series connection of several diodes. Further, capacitors increase energy losses by dissipation at switching.